Heretofore the prior art has proposed various systems, devices and apparatus for atmospheric condition detection, often referred to as "ice detection apparatus."
Among the various schemes heretofore proposed has been the development of systems which, in order to determine whether condensation formation is a possibility, attempt to determine relative humidity and the difference between ambient and a particular surface temperature. Various other prior art systems have also proposed the use of such humidistat type devices. Still other prior art devices rely on either chemical deposits or semiconductor material for moisture absorption. All of such prior art systems are not only costly but are very limited in accuracy and ambient temperature operating range and are thusly unsuitable to meet, for example, the present day requirements associated with the energy transmission and generating industry.
It is now proposed that extremely accurate and reliable atmosphere condition detecting systems can be constructed by employing a dew point determining stage which is based on energy absorption.
That is, light energy, as other forms of radiation, is emitted in small quantities called "quanta" and transmitted by electromagnetic waves. The production of light is attributed physically to actions taking place within atoms of the emitting source. Whitelight is in reality composed of many colors blended together. Such a whitelight beam, when refracted through a medium, such as a glass prism, disperses into a brilliant array, merging insensibly into one another and forming a spectrum.
Defining the spectrum requires specifying the vibration rate of the light source, or its corresponding wavelength. The relationship between these quantities is expressed by the equation: EQU C = f.sup.. w
where:
C = velocity of light PA1 f = frequency (vibration rate) PA1 w = wavelength PA1 ultraviolet . . . 400nM-130nM PA1 visible light . . . 400nM-760nM PA1 infrared . . . 760nM-50,000nM PA1 f= frequency PA1 w = wavelength PA1 c = velocity of light PA1 a. infrared and microwave regions (where absorption is due to water vapor molecule rotation alone); PA1 b. infrared and occassionally the visible regions (where absorption is due to water vapor molecule vibration and rotation); and PA1 c. visible and ultraviolet regions (where absorption is due to water vapor molecule electron changes together with vibration and rotation).
In the prior art, the best known application of the detailed study of emission and absorption spectra is in the use in identifying elemental particles within a sample of an unknown composition as by recognition of the characteristic spectra.
Generally, the electromagnetic spectrum is classified by wavelength as, for example:
Molecules may be considered as elastic arrays of electrically charged particles (atoms). Such particles, when subjected to electromagnetic radiation in the infrared region, will rotate or vibrate. Further, each specific molecule has a number of distinct frequencies of rotation and vibration.
When a molecule is subjected to infrared radiation of a frequency identical with one of its specific frequencies, it will be forced into vibration or rotation by absorption of energy from the infrared beam. Energy absorption measurements are recorded in terms of transmittance as a function of wavelength, with transmittance being defined as the energy (at a specific wavelength) emerging from a particular sample and expressed as a percentage of the energy entering that sample (also at the same wavelength). Accordingly, the percent transmittance as a function of wavelength is termed the infrared spectogram of the molecule, or, more commonly, its spectrum. Since the spectrum of each compound is unique, the infrared spectrum is often referred to as the "fingerprint" of a particular given compound.
Points of low transmittance in the spectra are termed absorption bands and the position of these bands (in the spectrum, depending on wavelength) identifies the compound while the depth of the absorption band can be related to concentration. Further, wavelength and frequency of infrared radiation are interrelated by the same formula: EQU C=f.sup.. w
where:
Since velocity is constant 3 .times.10.sup.10 cm/sec (speed of light) each frequency of energy absorption can be identified by the wavelength of radiation. Wavelength expressed in microns (1 micron = 10.sup..sup.-6 meters) has come to be the commonly accepted designation of regions of infrared spectrum, where specific molecules absorb energy.
The ability of a material to transmit infrared radiation is probably its most important characteristic.
Optical materials are used as windows or filters to admit infrared radiation, and as dispersing agents in the form of prisms to spread-out the radiation wavelength. Therefore, as employed by the invention, through the use of appropriate filters the desired infrared radiation can be obtained of any particular spectral span. This span, together with the selected spectral emission can be selected where water vapor absorption takes place.
Water vapor absorbs in the following broad spectral regions:
As set-forth above, a relatively large total spectral region yields a possible basis for moisture sensors; however, the characteristics of each spectral band have attendant limitations due to the equipment required. For example, in the untraviolet region where a strong water vapor absorption band exists at 200 nM, the emission of this wavelength requires high voltage supplies and expensive vacuum tube sources. The vacuum tube source also has a very short life since its operation depends on a particular gas emitted in the ultraviolet spectral region by virtue of extremely high voltage excitation.
Accordingly, the invention herein disclosed employs the infrared regions (because many absorption bands are present) and in application to the invention's sensor detector operation, the associated equipment is solid state requiring typically low level voltage supplies. As will become apparent, the invention can be practiced by employing the above characteristics and using, in combination, an infrared light emitting diode and a phototransistor to thereby create a very accurate sensor device.